Hemispheric processing of form versus texture at the local level of hierarchical patterns

Hemispheric specialization for global and local processing: A direct comparison of linguistic and non-linguistic stimuli

It is often assumed that the human brain processes the global and local properties of visual stimuli in a lateralized fashion, with a left hemisphere (LH) specialization for local detail, and a right hemisphere (RH) specialization for global form. However, the evidence for such global-local lateralization stems predominantly from studies using linguistic stimuli, the processing of which has shown to be LH lateralized in itself. In addition, some studies have reported a reversal of global-local lateralization when using non-linguistic stimuli. Accordingly, it remains unclear whether global-local lateralization may in fact be stimulus-specific. To address this issue, we asked participants to respond to linguistic and non-linguistic stimuli that were presented in the right and left visual fields, allowing for first access by the LH and RH, respectively. The results showed global-RH and local-LH advantages for both stimulus types, but the global lateralization effect was larger for linguistic stimuli. Furthermore, this pattern of results was found to be robust, as it was observed regardless of two other task manipulations. We conclude that the instantiation and direction of global and local lateralization is not stimulus-specific. However, the magnitude of global,-but not local-, lateralization is dependent on stimulus type.

Functional hemispheric differences for the categorization of global and local information in naturalistic stimuli

Up to now functional hemispheric asymmetries for global/local processing have mainly been investigated with hierarchical letters as stimuli. In the present study, three experiments were conducted to examine whether corresponding visual-field (VF) effects can also be obtained with more naturalistic stimuli. To this end, images of animals with a pattern placed on their body were displayed as stimuli. The task for the global level and for the local level was to categorize the animals and the patterns, respectively. As a result, VF-effects were also found for these stimuli and tasks. It is concluded that the hemispheric differences observed for hierarchical letters also hold for naturalistic stimuli.

Time-varying differences in evoked potentials elicited by high versus low spatial frequencies: a topographical and source analysis

OBJECTIVE

To investigate time-varying differences in visual-evoked potentials (VEPs) and dipoles elicited by high versus low spatial frequencies. The main question was whether different spatial frequencies are processed in distinct cortical areas, especially after 100 ms. An additional question was whether and how a hemispheric balance in spatial frequency processing develops over time.

METHODS

Stimuli were square-wave gratings, with spatial frequencies of 0.75, 1.5, and 6 c/d. VEPs and dipole models were analyzed at various latencies.

RESULTS

For the time-window of 80-100 ms, spatial frequency-related differences in VEPs and dipoles in posterior regions as reported previously were replicated: lower spatial frequencies were associated with more positivity in the VEP and with more anterior and radial sources than high frequencies. However, after 100 ms differences in amplitude, but not in topography and dipoles, were found between the different spatial frequencies. Between 180-200 ms a right hemisphere dominance was found for all frequencies.

CONCLUSIONS

After 100 ms, VEPs in response to different spatial frequencies seem to be generated in the same cortical areas. Also, no evidence for frequency-related hemispheric lateralization was found.

SIGNIFICANCE

Insight is provided into the functional-anatomical basis of longer-latency frequency-related differences in processing.

Interhemispheric interaction during global-local processing in mathematically gifted adolescents, average-ability youth, and college students

Interhemispheric interaction in mathematically gifted (MG) adolescents, average-ability (AA) youth, and college students (CS) was examined by presenting hierarchical letter pairs in 3 viewing conditions: (a) unilaterally to the right hemisphere (RH), (b) unilaterally to the left hemisphere (LH), or (c) bilaterally, with 1 member of the pair presented to each hemisphere simultaneously. Participants made global-local, match-no-match judgments. For the AA and CS, the LH was faster for local matches and the RH for global matches. The MG showed no hemispheric differences. Also, AA and CS were slower on cooperative compared with unilateral trials, whereas the MG showed the opposite pattern. These results suggest that enhanced interhemispheric interaction is a unique functional characteristic of the MG brain.

Hemispheric asymmetries for global and local processing as a function of stimulus exposure duration

The experiments assess the relationship between stimulus exposure duration and hemispheric asymmetries for global and local processing. Six durations (27, 40, 53, 67, 80, and 147 ms) were tested in a selective attention task in which hierarchical letterforms were presented unilaterally to the left or right visual field. The results indicated that left hemisphere-local asymmetries were demonstrated more often than right hemisphere-global asymmetries and that asymmetries were most commonly found in the middle range of durations tested. The pattern of results emphasizes the importance of further study into those conditions under which the predicted asymmetries can be reliably demonstrated in healthy individuals.